Canker frequently attacks the plant at the point of union. It is familiar to growers of Maréchal Niel, both out of doors and under glass; indeed, so common is it that many look for it as a natural course, much as they do for canker on a Ribston Pippin Apple, or for the collapse of branches on a Moorpark Apricot.
Canker frequently attacks the plant at the point of union, in the case of worked trees, but by no means always; attacks have been noted on almost all parts of the tree. The cause is not easy to trace. Members of the old school of pruners shake their heads at modem long pruning and prophesy canker, only to find their own lightly pruned trees suffer as badly, or worse.
In many cases the same predisposing cause as tends to canker in fruit trees, namely insufficient nourishment, operates, and in all cases where poverty of soil is suspected good soakings of liquid manure and a mulch should be supplied. Or a dressing of artificials may be given.
The canker may be cut away with a sharp knife or chisel, and the wound dressed with Stockholm tar.
That well-known Kentish rosarian, the Rev. H. B. Biron, has achieved successful results by making a slit in the bark 1/3 to 1/2 inch deep, right through the canker wound, beginning well above it and finishing well below. This is done in spring, at an early stage of the disease. The wound gapes, fresh bark forms, and the sap again flows strongly.